After about 1/2 dozen different clown and royal knifes, I found that 1 of my clown and 2 royals to be very aggresive when hungry, and they can definately hold there own against fish that are larger than themselves. Particulary at night time, I would come to the tank next day with to see tank mates with ripped fins and bite marks. Most of the time they spend hiding unless it's feeding time. I found all my knife fish ate everything I feed, and then some more. Definately don't have fish that are smaller than them. I would class their aggression level the same as arowanas but they do alot more damage than an arowana in a shorter period of time. I found that a low light on the tank at night help reduce the aggression. Don't let them get hungry if you have other fish in the tank.